Thermal spraying, also known as flame spraying, involves the melting or at least heat softening of a heat fusible material such as a metal or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. In a plasma type of thermal spray gun, a plasma stream is used to melt and propel powder particles. A high intensity arc within the gun heats and accelerates an inert gas such as argon or nitrogen to effect a plasma stream. An example of such a gun is described in U.S. Pat. No. 4,445,021. As shown in the patent, plasma guns typically are cooled by water channelling that direct the water over the electrode surfaces. Other types of thermal spray guns include a combustion spray gun in which powder is entrained and heated in a combustion flame, either at moderate velocity or high velocity (supersonic). In a wire type of gun a wire is fed through a combustion flame where a melted wire tip is atomized by compressed air into a fine spray for deposit. A two-wire arc gun melts contacting wire tips with an electrical arc for atomization by compressed air.
Although an aspect of thermal spraying is heating of the workpiece by the hot flame or plasma issuing from the gun and by the heated material depositing on the workpiece, the workpiece generally is maintained at relatively low temperature. The gun is traversed repetitively across the substrate being coated so as to distribute the coating particles and prevent local hot spots. Additionally, a jet or flow of coolant is impinged on the workpiece, away from the spray spot, to maintain temperature within several hundred degrees of room temperature. Cooling of the backside of a flat workpiece with air and water are disclosed respectively in U.S. Pat. Nos. 3,631,835 and 4,297,388. Excessive or uneven heating can result in oxidation of a metal substrate or cracking of a ceramic substrate.
Moreover, effective impingement of coolant onto a small workpiece can interfere with the spray stream and deposits. Cooling air must be very clean so as not to contaminate the coating. Also, spot cooling of cylindrical substrates is not very uniform, leading to stress. An example of a small cylindrical part to be coated is an oxygen sensor for pollution control in automotive engines, as disclosed in U.S. Pat. No. 4,265,930. The substrate in this case is formed of a tubular member of ceramic such as zirconium oxide. As is typical of automotive engine parts, low cost of production is an objective. Higher spray rate will achieve lower cost, but applies more heat to the substrate. This higher heat, even with conventional cooling methods, has resulted in cracking of the ceramic substrates.
Therefore, objects of the invention are to provide a novel cooling fixture and an improved method for cooling a tubular substrate, particularly a tubular ceramic substrate, during thermal spray coating thereof. Further objects are to provide a novel cooling fixture and an improved method for supporting and cooling a substrate for thermal spray coating of an oxygen sensor. Another object is to provide more uniform cooling of a tubular substrate. Yet another object is to allow thermal spraying of small tubular substrates at high spray rate.